1. Field of the Invention
This invention relates to a notch filter system which can be used to filter radio frequency communications signals.
2. Description of Related Art
A filter allows signals having certain frequencies to pass (pass band) while suppressing signals with other frequencies (attenuation band). The frequencies that separate the pass and attenuation bands are the cut-off frequencies. An ideal filter passes the pass band without attenuation and completely suppresses the attenuation band with sharp cut-off edges. In practice, typical filters attenuate the pass band somewhat, do not completely suppress the attenuation band and, at least at higher frequencies, do not have sharp cut-off edges. There are four general categories of filters related to the relation between the pass and attenuation bands: low-pass, high-pass, band-pass and band-stop. A notch or band stop filter passes frequencies below a frequency f1 and above a frequency f2 while suppressing frequencies between the frequencies f1 and f2.
A circulator is a ferrite device, i.e., a device that includes ferrite material. A typical ferrite component will include a compound of iron oxide with impurities of other oxides added. The iron oxide retains the ferromagnetic properties of the iron atoms while the impurities represented by the other oxides increase the ferrite's resistance to current flow. In contrast, elemental iron has good magnetic properties but a very low resistance to current flow. Such low resistance causes eddy currents and significant power losses at high frequencies. Ferrites, on the other hand, have sufficient resistance to be classified as semiconductors.
The magnetic property of any material is a result of electron movement within the atoms of the material. The two basic types of electron motion are the more familiar orbital motion (of the electron around the nucleus of the atom) and the less familiar electron spin (movement of the electron about its own axis). Magnetic fields are generated by current flow. The magnetic fields caused by the spinning electrons spin combine to give a material magnetic properties. In most materials, the spin axes of the electrons are so randomly arranged that the magnetic fields largely cancel out and the material displays no significant magnetic properties. But within some materials, such as iron and nickel, the electron spin axes can be caused to align by applying an external magnetic field. The alignment of the electrons axes within a material causes the magnetic fields to add together with the result that the material exhibits magnetic properties.
In the absence of an external force, the axis of spinning electrons tend to remain pointed in one direction in certain materials. Once aligned, the electrons tend to remain aligned even when the external field is removed. Electron alignment in a ferrite is caused by the orbital motion of the electrons about the nucleus and the force that holds the atom together, i.e., binding forces. When a static magnetic field is applied to the ferrite material, the electrons try to align their spin axes with the external magnetic force. The attempt of the electrons to balance between the external magnetic force and the binding forces causes the electrons to wobble on their axes. The useful magnetic properties of a ferrite is based upon the behavior of the electrons under the influence of an external magnetic field and the resulting wobble frequency.
Reciprocity is a term generally used to describe the transformation of a signal by a device. Fundamentally, if a signal S1 is input to a terminal T1 of a device and a signal S2 is output at a terminal T2 of the device, then the device is considered to be reciprocal if inputting a signal S2 at terminal T2 of the device yields the signal S1 on terminal T1 of the device. Ferrite devices are non-reciprocal devices. Such non-reciprocity is based upon Faraday rotation, in which a linearly polarized plane wave propagating through the ferrite material undergoes a rotation of its polarized direction independently of whether it is propagating in a forward or backward direction if the frequency of the propagating wave is much greater than the wobble frequency.
A circulator is more appropriately described as a non-reciprocal ferrite device. The cross-section of a ferrite device according to the Background Art is depicted in FIG. 1A. There, a circulator 100 includes a conductive launching disk 102 having terminals 104a, 104b, and 104c. Above and below the launching disk 102 are located ferrite disks 106a and 106b, respectively. Above the ferrite disks 106a and 106b are located permanent magnets 108a and 108b, respectively. The operation of the circulator 100 will be described in terms of corresponding FIGS. 1B and 1C.
FIG. 1B is the circuit diagram symbol for the circulator 100 of FIG. 1A. The circulator 100 provides unique transmission paths, allowing RF energy to pass in one direction (namely the rotation direction 110) with little (insertion) loss, but with a high loss (isolation) in the opposite (counter-clockwise) direction. The direction of rotation is determined according to the direction (perpendicular or anti-perpendicular) of the static magnetic field induced through the launching disk 102 by the permanent magnets 108A and 108B.
The direction of rotation 110 in FIG. 1B is clockwise. As depicted in FIG. 1C, if a signal is input to the circulator 100 at terminal 104a, then the signal will come out at terminal 104b. If a signal is input at terminal 104b, then the signal will come out at terminal 104c. And if a signal is input at terminal 104c, then the signal will come out at terminal 104a. 
If one of the terminals, e.g., 104c, is terminated with an impedance-matched load, then the circulator 100 functions as an isolator. The loaded terminal absorbs the energy passing to it. Hence, in the use of three-terminals, the isolator acts as a device that passes energy in one direction but not in the opposite direction.
A circulator/isolator can be constructed with 2 or more terminals, though a typical number of terminals is 3 or 4.
Wireless communications systems use both circulators/isolators and notch filters. Wireless communications systems include conventional cellular communication systems which comprise a number of cell sites or base stations, geographically distributed to support transmission and receipt of communication signals to and from wireless units which may actually be stationary or fixed. Each cell site handles communications over a particular region called a cell, and the overall coverage area for the cellular communication system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the system's coverage area.
When active, a wireless unit receives signals from at least one base station or cell site over a forward link or downlink and transmits signals to (at least) one cell site or base station over a reverse link or uplink. There are many different schemes for defining wireless links or channels for a cellular communication system, including TDMA (time-division multiple access), FDMA (frequency-division multiple access), and CDMA (code-division multiple access) schemes. In CDMA communications, different wireless channels are distinguished by different codes or sequences that are used to encode different information streams, which may then be modulated at one or more different carrier frequencies for simultaneous transmission. A receiver can recover a particular information stream from a received signal using the appropriate code or sequence to decode the received signal.
In the wireless communications industry, a service provider is often granted two or more non-contiguous or segregated frequency bands to be used for the wireless transmission and reception of RF communications channels. For example, in the United States, a base station for an “A” band provider for cellular communications receives frequency channels within the A (825–835 MHz), A′ (845–846.5 MHz) and A″ (824–825 MHz) bands, and the wireless units receive frequency channels within the A (870–880 MHz), A′ (890–891.5 MHz) and A″ (869–870 MHz) bands. A base station for a B band provider receives frequency channels within the B (835–845 MHz) and B′ (846.5–849 MHz) frequency bands, and the wireless units receive frequency channels within the B (880–890 MHz) and B′ (891.5–894 MHz) frequency bands. Additionally, a base station for a Personal Communications Systems (PCS) provider may receive frequency channels from wireless units on one or more PCS bands (1850 MHz–1910 MHz), and the wireless units receive frequency channels on one or more PCS bands (1930–1990 MHz).
A circulator can be used which has an operating band encompassing the frequency bands of operation to enable only a single antenna to transmit and receive, which can be referred to as duplex operation. The circulator can be arranged such that signals being transmitted enter into a first terminal of the circulator and are output at a second terminal to the antenna. Signals received at the antenna can be input into the second terminal and produced at a third terminal to the receiver circuitry.
Filters are used to prevent energy from one frequency band from interfering with another frequency band. Here, the frequency band can be narrower than the frequency bands described above or wider. For example, the frequency band can be a 1.25 MHz wide CDMA loaded carrier or a 5 MHZ wideband CDMA loaded carrier within the frequency bands described above. However, due to the finite roll-off characteristics of filters in the radio receiver, a signal from an adjacent band may come through the radio receiver at a power level strong enough to interfere with an adjacent band. To help prevent this, guard bands are used to space the carrier frequency bands apart. However, the use of guard bands removes bandwidth which can be used to transmit actual communications signals.